Detection of chemicals with infrared light

ABSTRACT

The present invention is directed to a method for non-contact or stand off chemical detection by selectively exciting one or more analytes of interest using an IR source tuned to at least one specific absorption band without significantly decomposing the analyte and determining if the analyte is present by comparing emitted photons with an IR detector signal made before and during or shortly after exciting the analyte. Another embodiment provides a method for non-contact or stand off chemical detection by selectively exciting one or more analytes of interest using an IR source tuned to at least one specific absorption band without significantly decomposing the analyte, wherein the analyte is excited sufficiently to generate a vapor plume, and wherein the plume is examined to detect the presence of the analyte. Additionally, the present invention provides for a system for non-contact or stand off chemical detection.

PRIORITY CLAIM

The present application is a continuing application of U.S. applicationSer. No. 13/330,728 filed on Dec. 20, 2011 by R. Andrew McGill et al.entitled “Detection of Chemicals with Infrared Light,” which was acontinuing application of U.S. application Ser. No. 12/255,103 filed onOct. 21, 2008 by R. Andrew McGill et al. entitled “Detection ofChemicals with Infrared Light” (issued as U.S. Pat. No. 8,101,915 onJan. 24, 2012), which claimed priority from U.S. Provisional ApplicationNo. 61/135,531 filed on Oct. 24, 2007 by R. Andrew McGill et al.,entitled “Method of Chemical or Biochemical Detection,” the entirecontents of each are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to chemical or biochemicaldetection and more specifically to molecular or ion detection byselective thermal heating with incident infrared light resonant with oneor more selected infrared absorption bands, and examination of vapor orlight produced as a result of said incident light.

2. Description of the Prior Art

Low vapor pressure analytes, in their condensed physical state, such asexplosives, drugs of abuse, and chemical warfare agents, are difficultto detect in a non contact or stand off mode, using conventionaltechnology. Since the handling of these materials leaves persistenttrace contaminations on contacted surfaces (e.g., door handles, pockets,hands, containers, etc.), manual swipes are often used to lift particlesor residue of the analytes for subsequent analysis, for example inairport hand luggage screening. However, performing a manual swipe ofeach potential suspect and container is often impractical orinconvenient, and it inhibits covert detection.

A summary of existing and emerging technologies for the portabledetection of contaminants, such as chemical warfare agents, is outlinedby Michael W. P. Petryk in his article entitled “Promising SpectroscopicTechniques for the Portable Detection of Condensed-Phase Contaminants onSurfaces,” found in Applied Spectroscopy Reviews, 42: 287-343, 2007, theentire contents of which are incorporated herein by reference. Moreover,U.S. Pat. No. 6,998,156 to Bubb et al. describes using an infrared laserto vaporize target material. The entire contents of the Bubb patent areincorporated herein by reference.

For trace detection of explosives or drugs, the current technologiestypically work well as long as particles can be collected by somephysical means and then thermally converted into vapor for analysis ordetection. The sampling techniques use non-selective removal ofparticles from a selected few surfaces which may have been contaminatedwith particles of explosives and transfer them onto a heated surfacewhich is interfaced to an ion mobility spectrometer (IMS) or otherexplosive detection system (EDS). Previously, this has required either aphysical rubbing process or forced air removal, neither of which ismaterial selective or practical for any significant stand off distanceand can add a significant time and personnel cost burden to thedetection process. In addition, the efficiency of physical removal ofparticles from a surface as particles depends on the techniques used,the training level of the person removing the particles, and the rubbingor contaminated surface material or surface roughness of contaminatedsurfaces.

Known methods of heating trace samples of explosives for detectionpurposes include broad band IR sources which heat in a non selectivefashion. This approach consumes much more power than a selective heatingprocess and generally heats everything incident with the heating source.This increases the general background level in the vapor phase of allthe volatile chemicals in the material examined and can result in anincrease in signal clutter or false alarms, especially when thesubstrate materials or additional contaminants being examined are of acomplex natural origin such as leather, wood or food products.

Laser induced breakdown spectroscopy (LIBS) is an alternative laserbased technique but this requires significantly higher power and resultsin the destruction of the sample of interest and the surface on which itdirectly resides. The lasers used for LIBS are typically high power (10mJ or greater) with short wavelengths (UV to near IR) and are notconsidered safe for environments where humans might be exposed or forthe integrity of the substrate being examined. LIBS is a type of atomicemission spectroscopy which utilizes a highly energetic laser pulse asthe excitation source to ablate material, reducing it to its elementalconstituents. LIBS can analyze any matter regardless of its physicalstate, be it solid, liquid or gas. Because LIBS detects elements, itsselectivity in the presence of many materials is suspect and is relianton signal ratios of elements which can be confused when mixtures ofmaterials are present. Nitrogen, for example, is present in manyexplosives but it is also prevalent in cotton or wool fiber or anyproteinacous material. Trace explosives present on natural fibers wouldbe difficult to detect accurately with LIBS.

Raman spectroscopy is an emerging standard for optical identificationand characterization of known and unknown samples. It couples tosignature vibrational modes of the analyte and is complementary toinfrared spectroscopy. Its main drawback is in its inefficiency becausetypically only one photon is Raman scattered for every million photonsincident on the sample. Furthermore, Raman is isotropic, meaning thereis no preferred direction for the scattered light to travel. This limitsits application for stand off detection. For a fixed collection opticdiameter, the photon collection efficiency decreases proportional to thesecond power of the distance to the sample under interrogation. Finally,Raman efficiency is optimized with high photon energy light which is noteye-safe to use in the presence of people.

Photo-thermal spectroscopy is another potential tool that is used instand-off detection. In this technique, the sample is heated with anon-resonant, not eye-safe laser (usually visible wavelength of near-IR)in a periodic fashion (using a mechanical chopper). The detected signalconsists of the amplitude of the heated signal measured by an IRdetector (or some other means) and its phase-angle shift with respect tolaser heating. This method differs from the present invention, in part,because it does not take advantage of the resonant nature of absorptionof IR radiation which allows analyte selectivity right at the excitationstage and with much less laser power to achieve suitable heating.

One method of detecting explosives uses a broadband heating sourceconnected to an IMS. One problem with this method is that the entirecomposition of the surface, and possibly deeper, is heated which makesaccurate detection of the analyte more difficult. Another method ofdetecting explosives, narcotics and other chemical substances, uses alaser source to ablate the particles, then collects them andsubsequently analyzes them. Unfortunately, the ablation process maydamage the analyte, resulting in additional signal clutter and possiblereduction in the principle analyte signal, and this method requires aseparate collection step.

BRIEF SUMMARY OF THE INVENTION

The aforementioned problems are overcome in the present invention whichprovides a method for non-contact or stand off chemical detection thatmay be eye-safe by selectively exciting one or more analytes of interestusing an IR source tuned to at least one specific absorption bandwithout significantly decomposing the analyte and determining if theanalyte is present by comparing emitted photons with an IR detectorsignal made before and during or shortly after exciting the analyte.Another embodiment of the present invention provides a method fornon-contact or stand off chemical detection that may be eye-safe byselectively exciting one or more analytes of interest using an IR sourcetuned to at least one specific absorption band without significantlydecomposing the analyte, wherein the analyte is excited sufficiently togenerate a vapor plume, and wherein the plume is examined to detect thepresence of the analyte. The analyte of interest may be an explosive, anadditive to an explosive, a drug, a chemical warfare agent, abiochemical, or a biological warfare agent. Additionally, the presentinvention provides for a system for non-contact or stand off chemicaldetection that may be operated in an eye-safe manner.

The present invention has many advantages over the prior art. It maydetect at stand off distances and be safely pointed at targets in areaswith people present. It may selectively target materials in a complexmatrix. It may simultaneously detect a range of materials (e.g.,explosives, drugs, and chemical agents). It may detect at video framerates or faster. It may be hand held. It may be operated without thepersons present being cognizant of the ongoing detection process. It mayefficiently vaporize or probe particles that are embedded in a surfacesuch that they are difficult to remove using the normal physical rubbingor particle removal techniques.

For the stand-off detection of explosives and devices such as IEDs,photonic detection has several inherent advantages compared to othermethods. These advantages include: an extremely high detection speed,zero interference with the existing radar and communication systems, andthe potential for long range stand-off sensing. While several otherall-optical techniques such as LIBS and Raman have been proposed forstand-off detection, these suffer from the fact that the wavelengths andintensities required are not safe to eyes and skin, or even to surfacesexamined such as painted automobiles.

By using more than one laser (multispectral mode) or even a continuouslytunable laser (hyperspectral mode) further selectivity can be achievedby using a combination of resonant and/or non-resonant excitation. Onthe collection side, further selectivity can be achieved by detectingonly portions of the thermal band that is characteristic to the analyteof interest. Furthermore, the present invention uses laser sources (e.g.QCLs) that are small, provide light that is invisible and safe to thehuman eye and are conducive to implementation in hand-held devices.Also, the transient response of the analyte to laser heating is measuredinstead of a response to periodic heating, thus providing a means forrapid detection as well as circumventing the problems associated withdetection of loose powders/particles (such as explosives particulateresidues) that are hard to detect using photothermal imaging due to longthermal constants. Also, the use of resonant infrared laser sources whenproperly focused to a small spot (to increase laser irradiance) cancause the analyte to partially or completely vaporize and be detected bysophisticated IMS detectors or other suitable vapor detectors which maybe optically based and probe the vapor generated in concert with thevaporization process to allow interrogation during the time period wherethe concentration of vapor is high, before the vapor diffuses or movesby other means to a lower concentration condition. This embodiment ofthe present invention may be non-contact or stand-off, and providesadvantages over currently available detection methods, especiallycompared to contact techniques that involve physical rubbing or airjetting of the substrate to remove solid particles of material toexamine.

These and other features and advantages of the invention, as well as theinvention itself, will become better understood by reference to thefollowing detailed description, appended claims, and accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic for three embodiments of the present invention:(a) shows detection using an IR source and an IR detector; (b) showsdetection using an IR source and a vapor detector; and (c) showsdetection using an IR source, a probing light source, and a detectionsystem.

FIG. 2 shows the chemical structure of nitroglycerine,2,4,6-Trinitrotoluene (TNT), and RDX.

FIG. 3 shows IR transmission spectra of humid air, 2,4-dinitrotoluene(24DNT), TNT, and RDX. The N—O absorption stretching mode at 1600 cm⁻¹(6.25 microns) falls in a fortuitous transparent window for air. Alsohighlighted are shared absorption bands at 7.41 μm, 11.0 μm, 12.62 μmand 13.65 μm that fall in an extended air transparency window.

FIG. 4 shows the IR spectral absorbance properties of humidified air.

FIG. 5 shows an infrared spectrum for 2,3-dimethyl-2,3-dinitrobutane(DMNB) vaporized with an IR laser coupling to the N—O stretch. The IRlaser tuned to 6.45 microns instantly evaporated the DMNB, and thecollected material was shown to be essentially spectrally identical tothe starting material.

FIG. 6 shows a stroboscopic back-illuminated photo of a solid piece of24DNT directly after exposure to a laser beam tuned to 6.25 microns. Thevaporized 24DNT is clearly visible as a plume above the solid 24DNT 14milliseconds after the laser pulse. Spectroscopic examination of 24DNTbefore and 24DNT after vaporization and collected on a suitable plateconfirm that the 24DNT is not decomposed during the vaporizationprocess.

FIG. 7 is a thermal image of RDX deposited as a trace quantity on apolyethylene substrate illuminated with a QCL with incident light at 6.3microns. The brighter area in the central part of the figure indicatesthe presence of the explosive.

FIG. 8 shows thermal images of RDX deposited as a trace quantity on agold minor substrate illuminated with a QCL with 5 mW/mm² incident lightat 6.3 microns. FIG. 8( a) is an image with the laser off, 8(b) is animage with the laser on, and 8(c) is a differential image which clearlydefines the RDX location and spatial distribution in the center.

FIG. 9 shows an image of a transparent plastic substrate with RDX andTNT deposited as the horizontal and vertical letters RDX and TNTrespectively and illuminated by a heatgun but not with a laser.

FIG. 10 shows IR transmission spectra of RDX and TNT highlighting thefrequencies used in Example 4.

FIG. 11 shows an image of a transparent plastic substrate with RDX andTNT deposited as the horizontal and vertical letters RDX and TNTrespectively and illuminated by a laser at a wavelength not selective orabsorbed for either RDX or TNT.

FIG. 12 shows an image of a transparent plastic substrate with RDX andTNT deposited as the horizontal and vertical letters RDX and TNTrespectively and illuminated by a laser at a wavelength resonant to theTNT structure.

FIG. 13 shows an image of a transparent plastic substrate with RDX andTNT deposited as the horizontal and vertical letters RDX and TNTrespectively and illuminated by a laser at a wavelength absorbedsignificantly to both the RDX and TNT structures.

FIG. 14 shows an image of a transparent plastic substrate with RDX andTNT deposited as the horizontal and vertical letters RDX and TNTrespectively and illuminated by a laser at a wavelength resonant to theRDX structure.

DETAILED DESCRIPTION OF THE INVENTION

According to the present invention, low vapor pressure analytes (e.g.,explosives, additives to explosives, drugs, chemical warfare agents,biochemicals, and biological warfare agents) that are typically presentas solid particles or as a thin film residue under ambient conditionscan be detected at non contact or stand off distances. After someonehandles illicit materials, particles of explosives or drugs areunwittingly transferred through fingerprints onto objects and surfacesthat the person touches, or after a release of a chemical agent, thedisseminated chemical is distributed onto a variety of surfaces as atrace residue. The contaminated object or surface can be excited (e.g.,heated) actively and selectively by using an IR laser or a filteredlight source so that a narrow wavelength range is used to be resonantwith one or more selected absorption band(s) of the analyte of interest.Selective heating is used to maximize heating of the analyte of interestand to minimize heating or potential damage to materials that are not ofinterest to the detection application (e.g., substrate, contaminants),with the added benefit that significantly less laser power is needed.The laser is coupled in a resonant fashion to one or more selectedinfrared absorption band(s), with a wavelength between 1 and 20 microns,to maximize the efficiency of energy transfer and to avoidelectronically excited states that commonly lead to decompositionproducts. Typically, by selectively exciting the analyte using an IRsource tuned to a specific absorption band, no more than five percent ofthe analyte is decomposed.

Detecting the analyte of interest may be accomplished by using anyappropriate analytical tool. Examples include examining thermal or photoemissions with an IR detector and examining a vapor plume generated bythe heating. Using thermal or photo emissions, signals, such as images,of a given object or surface are used to detect the analyte of interest.An image taken before heating is compared to an image during heating.The difference between the images or a differential image created bysubtracting the image taken before from the image taken during heatingcan identify the presence of the analyte of interest. When sufficientlaser energy is used to generate a significant amount of vapor, thevapor plume can be analyzed by various optical means that are separatefrom the original IR heating source. This could be achieved via a lightabsorption phenomena or LIDAR or cavity-ringdown techniques.Alternatively, the generated vapor plume could be analyzed by aninstrument like an IMS or a technique like gas chromatography.

FIG. 1 shows a schematic for three embodiments of the present invention.FIG. 1( a) shows detection using an IR source and an IR detector. FIG.1( b) shows detection using an IR source and a vapor detector. FIG. 1(c) shows detection using an IR source, a probing light source, and anoptical detection system.

The present invention provides a means to detect low vapor pressureanalytes, such as explosives, drugs, and chemical agents, based onresonant absorption of certain infrared (IR) wavelengths. Some examplesof explosives or components of explosives that may be detected include24DNT, TNT, RDX, HMX, TETRYL, PETN, NG, EGDN, DMNB, ammonium nitrate,urea nitrate, ANFO, TATP, and H₂O₂. Some examples of drugs that may bedetected include heroin, cocaine, barbiturates, LSD, and cannabis. Someexamples of chemical agents that may be detected include nerve (G),blister (H), blood, incapacitating, and lacrymator. This system can beapplied to virtually any material, including any explosive type,including organic or inorganic (ionic) explosives that cover thetraditional (e.g. TNT, RDX, PETN, and ANFO) and non traditionalmaterials (e.g. triacetonetriperoxide or TATP and H₂O₂) solid or liquidstates, and it can be used to simultaneously detect all organicenergetic materials probed at a common wavelength and containing thenitrogen-oxygen bond (N—O), which is in the majority of the commonlyused explosives (see FIG. 2). Likewise, for the inorganic nitrates andperoxide based explosives, the system can be tuned to a wavelength at ornear a peak in the absorption spectrum characteristic of the nitrate ionand peroxide structures respectively. To detect the class A drugs, theIR source can be tuned to a wavelength at or near a peak in theabsorption spectrum characteristic of the carbon-nitrogen bond. Inaddition to low vapor pressure analytes, the present invention is alsocapable of detecting higher vapor pressure analytes.

The N—O band in the nitro group has natural resonance frequencies(symmetric and anti-symmetric) in the mid and long IR wavelengths. Thetransmission spectra of organic based 24DNT, TNT and RDX chemicals showsthat they exhibit a common absorption band near 6.25 μm as shown in FIG.3. This N—O stretch band and several others fortuitously fall intransmission windows of air (see FIG. 4), making them suitable for inair stand-off detection applications.

Because the vapor pressures of the majority of explosives are very lowand typically well below a few parts per trillion at room temperature,traditional optical stand-off detection techniques applied to plumes ofindustrial chemicals or chemical agents are largely not useful.Additionally, the vapor pressure of explosives can be significantlyreduced in composites containing explosives, such as C4, and masked dueto complex packaging. However, the surface contamination of explosivesand this persistence in the solid state can be exploited for stand-offor non-contact detection by irradiating the explosive sample with one ormore of the resonant absorption wavelengths highlighted in FIG. 3. Atthese wavelengths, the coupling efficiency of the optical energy can be1000 times greater than if the wavelength were just a few cm⁻¹ away.Targeting any of these absorption peaks enables rapid, selective heatingof the explosive material.

As the temperature of an object increases, the amount of IR radiation itemits increases, which allows IR imaging to map out local temperaturechanges. Industrial, medical, and military applications make use of thistechnology, and, because of good atmospheric transmission at somewavelengths, remote imaging is also possible. A trace amount ofexplosive residue, when illuminated at a resonant IR absorptionwavelength, will heat up locally and is observable by an IR camera.

If the thermal properties of the substrate on which the analyte is foundare known, then the knowledge of both the degree of heating and thecooling time can be used as inputs in an alarm algorithm. For example,more heating at selected wavelengths on a metallic substrate such as adoor knob (fast cooling) carries more weight (i.e. more likely tocontain explosive residue) in an alarm algorithm than the same amount ofheating on a plastic or fibrous substrate (slow cooling).

Under one embodiment of the present invention, IR thermal imaging of agiven scene is enhanced by illumination with an IR pulse (outside thecamera detection wavelength range) to resonantly interact with theanalyte or analytes of interest. By comparing images with the laser offto images with the laser on at video frame rates, a differential imagewith high fidelity can be generated which will distinctly identifyexplosive residue.

The resonant IR absorption wavelengths to use for heating should becommon to explosives, but otherwise rare among possible substratematerials such as cotton, paper, plastics, metals etc. Further, byutilizing additional IR wavelengths, tests can be performed to probe forother types or classes of explosives as well as to increase systemselectivity to confirm the type of explosive. In general, this approachoffers advantages over other optical techniques in that it is eye-safeand compact.

Thermal imaging is enhanced by the heating signature due to the resonantabsorption within the explosive residues of interest. For thisembodiment, powerful mid-IR laser sources and sensitive IR focal planearrays may be used. Both of these are commercially available today andshould offer increased performance in upcoming years.

The IR source may be any source known in the art, such as a pulsedlaser, continuous laser, broad band light source, filtered broad bandlight source, swept source, chirped source, variable source, or tunablesource. Preferably, a quantum cascade laser (QCL), may be used as the IRphoton source. The advantages of using QCL include: It can provide asingle wavelength output allowing for the targeting of specificfunctional groups. It can operate at room temperature, and currentdevices can provide up to 1 Watt CW output, and it is commerciallyavailable. Moreover, a pulse is preferred over continuous wave (CW) forhigher peak power, lower laser on times, and reduced coolingrequirements. It is a stable laser source which in normal operationrequires no consumable materials. It is microfabricated fromsemiconductor wafers to generate die, in mass production, with highyield, low cost and high quality. The inexpensive and miniature natureof the QCL die are attractive for the development of hand heldinstrumentation. At present, λ=6 μm QCLs have been demonstrated incontinuous wave (CW) mode up to T=340 K (63° C.). The device can output640 mW CW at room temperature and greater than 1.6 Watts when cooled to100 K. These output powers in a laser tunable to the absorption bands ofexplosives makes the QCL an enabling technology for compact detectorsused in non contact or stand off vaporization and detection ofexplosives.

Any thermal imaging hardware may be used. For example, an IR camera(such as a commercial FLIR camera) can be used to collect and analyzelongwave infrared (LWIR) light. The advantages of this hardware include:it is a microfabricated bolometer thermal imaging array, it is uncooledfor low power operation, it responds to the 7-12 μm band which includeswavelengths generated from thermal heating, and it is small andlightweight. Moreover, a telescopic lens may be used to increase standoff distance capability.

The IR detector may be a single channel or a multi channel detector.Also, the light entering the IR detector may be filtered to be selectivefor the analyte of interest.

In accordance with Kirchhoff's law, the thermal emission spectra ofanalytes correlate with their absorption spectra. To take advantage ofthe increased emissivity levels at absorption peaks in the thermal bandwhere the detection occurs, a suitable optical filter can be designedwhich passes these analyte specific wavelengths and blocks all others.This way, only a signal attributable to the analyte is collected and thesignal-to-noise ratio is increased. By combining both selectiveexcitation and selective collection, the detection limit and detectionselectivity of a given analyte can be greatly increased.

If there is sufficient IR light to generate a vapor plume, an opticaltechnique may be used to examine said plume. The vapor may be examinedusing an IR source, visible source, UV source, IR detector, visibledetector, UV detector, diffraction grating, filter wheel, Michelsoninterferometer, laser cavity, integrating sphere, or any other suitableequipment. The presence of the analyte may be determined by using IRabsorption, IR backscattering, IR thermal luminescence, Ramanspectroscopy, LIBS, or any other suitable technique.

If a vapor plume is generated, it can be detected by any means of vapordetection currently available or yet to be discovered. These include ionmobility spectrometry, mass spectrometry, gas chromatography,chemiluminescence, surface acoustic wave, gravimetric, microbalance, orcrystal resonance.

The present invention can be inherently eye-safe, with anticipated IRirradiances that are far below the maximum permissible exposure limit,which is of the order of 100 mW/cm². As such, many uses can beenvisioned for this system, such as to not only scan suspected IEDs, butalso to scan people (including for example: clothing, skin, glasses,shoes, hat, hair), airline boarding passes, vehicles, luggage, parcels,etc. Anything that a person handling explosives contacts is a suitabletarget.

Two or more lasers may be used to increase sensitivity and selectivityor expand the range of analytes examined. Multiple lasers used in on-and off-resonance modes would improve the selectivity of the system byremoving false positives which may occur because sometimes broadbandabsorbers heat independent of wavelength. Looking at the differencebetween the signals for on- and off-resonance with neighboringwavelengths will help remove effects from materials which are broad bandIR absorbers and prevent false positives. If the difference between thetwo differential signals is negligible, the analyte of interest isconcluded to be not present. To compare the difference between signalson- and off-resonance, compare either the difference of the differentialsignals or just the difference between the on- and off-resonance rawsignals. The nature of the algorithm applied depends on the substratematerial being examined.

The present invention is applicable in mobile and static applications.For mobile applications, the laser may be trained on a target so that asthe detector/laser is moved the laser and camera remains trained on thetarget of interest. For targets which are mobile a static or mobilelaser and camera can similarly be trained on the target of interest.

Another alternative is to use single element photo-conductive infrareddetectors (such as mercury-cadmium-telluride). The advantage of thisalternative is higher sensitivity (allowing longer stand-off distances),increased detector speed (allowing faster moving objects to be scanned)and incorporation of a less expensive detector. Also, higher detectorspeeds will allow the probing of the thermal pulse immediately afterresonant heating (sub millisecond range) before the heat is transferredto the substrate (so called thermal luminescence regime), thus furtherdecreasing the contribution of the substrate on the total thermalsignature. This approach is likely to include a visible-light camera orvisible optic with a cross-hair reference in the user interface foraiming the instrument at the desired measurement target.

EXAMPLE 1

A small sample of dimethylnitrobutane (DMNB) was positioned in arecessed stainless steel support inside a chamber under reduced pressureand positioned in line with a laser beam tuned to 6.45 microns. DMNB isa taggant that is added to formulation of explosives and has the samefunctional groups as some conventional explosives. It was selected forthis experiment as a simulant or surrogate for an actual explosive butwith very similar spectral properties. The laser used was a tunablefree-electron laser that allows the user to select a wide range ofwavelengths. The operational characteristics of the laser were asfollows: wavelength of 6.45 microns, 6 microsecond pulse train of 1 pspulses at 2.8 GHz, energy density of 30-40 micro J/cm².

Immediately after directing the laser beam onto the DMNB target avisible plume of material was ejected. Some of this material wascollected on a neighboring substrate for characterization by FTIR. Asshown in FIG. 5, the collected material had the same spectral signatureas the starting DMNB target material, proving that the laser energy wascoupled into the DMNB without any significant chemical degradation. Froma visual examination of the plume and the topography of the collectedmaterial, both vapor and particulate matter were ejected from the DMNBtarget. The laser intensity required to achieve these results was 30-40microJ/cm².

EXAMPLE 2

A small intrinsic sample of 2,4-dinitrotoluene (24DNT) positioned on atest surface under ambient laboratory conditions was exposed to a laserbeam tuned to 6.25 microns. 24DNT is a natural degradation product ofand an impurity in TNT that is structurally very similar to TNT with oneless nitro group. It was selected for this experiment as a closesurrogate for TNT with very similar spectral properties which wasdesirable for the intended tests. The laser used was a tunablefree-electron laser that allows the user to select a wide range ofwavelengths.

The FEL tuned to 6.25 microns with 14 mJ pulses readily heated andpartially vaporized the 24DNT slab (1 cm long) and 14 ms after a pulsefrom the laser a plume emitted is visible above the solid target. Asequence of still stroboscopic back-illuminated photos was collected inclose sequence to generate a video recording the effects of the laser onthe sample of 24DNT. One of the still photos is shown in FIG. 6. Some ofthe ejected material was collected on a neighboring substrate forfurther examination. Post analysis of the collected material confirmedthat it had the same FTIR spectrum as the 24DNT target material. Thelaser energy was coupled into the 24DNT without any apparentdecomposition.

EXAMPLE 3

A small trace sample of RDX deposited separately on planar polyethyleneand gold substrates was positioned (under ambient laboratory conditions)in line with a quantum cascade laser (QCL) beam with an outputwavelength of 6.30 microns. The laser was focused to a spot size on thetarget of 1-2 mm². To examine any light that was emitted from the RDXsample after exposure with the QCL, an infrared camera (Photon Block 2from FLIR, sensitive to light in the 7-12 micron range) was positionedto examine any infrared light received back from the test substrate. RDXis a common explosive in land mines and other military ordnance. The QCLallows a significant output power (>50 mW) to be achieved at the desiredwavelength.

FIG. 7 shows the thermal image of RDX deposited as a trace quantity on apolyethylene substrate, illumination with QCL at 6.3 microns (5 mW/mm²).Immediately after directing the laser beam onto the RDX target, infraredlight was detected indicating that the sample was being heated. Whenusing a polyethylene substrate, the RDX heating was visible even in thebackground of reflected light or other emitted light. In photos taken ofthe sample (where black is low intensity and white is high intensity),the regions of the substrate where RDX was present appeared lighter whenthey were heated due to the resonant absorption of the QCL beam. Regionsof the substrate where there was no RDX remained dark, indicatingambient temperature. When the laser was off, the entire region of theimage was dark. Rapid RDX heating at 1 foot of stand off is clearlyseen.

For the gold mirror substrate, the thermal conduction away from the RDXsample is higher. It is possible to see the thermal heating byinspection of the raw collected infrared image, but it was not as clearas the image collected on the polyethylene substrate. To enhance theimage, a sequence of video frames was collected and a differential imagewas computed by subtracting the image directly before turning on the QCLwith frames after turning on the QCL. Using this differential imagingapproach, much clearer thermal image pictures were obtained to identifywhere the RDX was located, as shown in FIG. 8.

The above experiment was repeated with the same RDX depositedpolyethylene target sample but the RDX was illuminated with a defocusedspot size of 100 mm². The results of this experiment showed that theexplosives were visible over a footprint range of a square centimeter.

By traversing or rastering the laser over the RDX deposited surface, theRDX could be mapped out over the entire surface examined. The QCL lightat 6.3 microns efficiently coupled into the RDX sample and thermallyheated the sample by a few degrees, which was sufficient to generate IRlight from the RDX. The thermal image was captured with an uncooledmicrobolometer array IR detector (FLIR Photon Block II). The thermalheating of the RDX sample was very rapid and occurred within the timeframe of the IR video used. At 30 frames/s, this indicates thatsignificant thermal heating occurred in <30 ms. Cooling after the laserexposure ends may occur over a longer time period. The cooling rates forthe RDX (and other analytes) can aid in identifying the trace explosivematerial.

EXAMPLE 4

Selectivity was tested for a dual analyte sample containing RDX and TNT.RDX was put horizontally on a transparent plastic substrate in the formof the letters R, D, and X. Similarly, TNT was put vertically on thesame stainless steel surface in the form of the letters T, N, and T.FIG. 9 shows the sample illuminated by a heatgun with no laser. FIG. 10shows the frequencies that were used in the example: ν₁ wasoff-resonance for both TNT and RDX, ν₂ was on-resonance for TNT but notRDX, ν₃ was on-resonance for both RDX and TNT, and ν₄ was on-resonancefor RDX but not TNT. As shown in FIG. 11, no image appeared when thelaser was off-resonance. When on-resonance for TNT but not RDX, only theTNT letters were significantly visible (FIG. 12). When on-resonance forboth RDX and TNT, both TNT and RDX letters were visible (FIG. 13).Finally, when on-resonance for RDX but not TNT, only the RDX letterswere significantly visible (FIG. 14). The circles in the figuresindicate the laser spot size.

The above descriptions are those of the preferred embodiments of theinvention. Various modifications and variations are possible in light ofthe above teachings without departing from the spirit and broaderaspects of the invention. It is therefore to be understood that theclaimed invention may be practiced otherwise than as specificallydescribed. Any references to claim elements in the singular, forexample, using the articles “a,” “an,” “the,” or “said,” are not to beconstrued as limiting the element to the singular.

What is claimed as new and desired to be protected by Letters Patent ofthe United States is:
 1. A method for non-contact chemical detection,comprising selectively heating one or more condensed-phase ionicanalytes of interest present on a substrate using an IR source tuned toat least one analyte-specific absorption band, wherein the analyte isheated sufficiently to liberate a dissociated ionic analyte as vapor orgaseous material, and wherein the liberated vapor or gas is examined todetect the presence of the analyte.
 2. The method of claim 1, whereinthe method is eye-safe, and wherein the method does not causesignificant damage to the substrate.
 3. The method of claim 1, whereinthe method is stand off.
 4. The method of claim 1, wherein the analyteof interest is selected from the group consisting of explosives,additives to explosives, drugs, chemical warfare agents, biochemicals,and biological warfare agents.
 5. The method of claim 1, wherein the IRsource is a pulsed laser, continuous laser, broad band light source,filtered broad band light source, swept source, chirped source, variablesource, or tunable source.
 6. The method of claim 1, wherein multiple IRwavelengths are used to detect multiple analytes of interest.
 7. Themethod of claim 1, wherein multiple IR wavelengths are used to improveselectivity where at least one IR wavelength is on-resonance for theanalyte and at least one IR wavelength is off-resonance.
 8. The methodof claim 1, wherein the IR source emits somewhere in the range between 1and 20 microns.
 9. The method of claim 1, wherein to detect nitrogenbased explosives, the IR source is tuned to a wavelength at or near apeak in the absorption spectrum characteristic of an N—O bond; whereinto detect phosphonate ester chemical warfare agents, the IR source istuned to a wavelength at or near a peak in the absorption spectrumcharacteristic of a P═O double bond or a C—O—P bond; and wherein todetect drugs of abuse, the IR source is tuned to a wavelength at or neara peak in the absorption spectrum characteristic of a secondary ortertiary amine.
 10. The method of claim 1, wherein an optical techniqueis used to examine the vapor plume.
 11. The method of claim 1, whereinthe vapor plume is examined using an IR source, visible source, IRdetector, visible detector, diffraction grating, filter wheel, Michelsoninterferometer, laser cavity and integrating sphere, or any combinationthereof; and wherein the presence of the analyte is determined by usingIR absorption, IR backscattering, IR thermal luminescence, Ramanspectroscopy, LIBS, or any combination thereof.
 12. The method of claim1, wherein the vapor plume is examined by ion mobility spectrometry,mass spectrometry, gas chromatography, chemiluminescence, surfaceacoustic wave, gravimetric, microbalance, crystal resonance, or anycombination thereof.
 13. A system for non-contact chemical detection,comprising: (a) an IR source tuned to at least one analyte-specificabsorption band specific to one or more analytes of interest; and (b)means for detecting the presence of the analyte; wherein, the analyte isa condensed phase ionic material present on a substrate and isselectively heated using the IR source liberating one or moredissociated species as vapor or gaseous material, and the presence ofthe analyte is determined by examining the liberated vapor or gas. 14.The system of claim 13, wherein the detection is eye-safe, and whereinthe detection does not cause significant damage to the substrate. 15.The system of claim 13, wherein the detection is stand off.
 16. Thesystem of claim 13, wherein the analyte of interest is selected from thegroup consisting of explosives, additives to explosives, drugs, chemicalwarfare agents, biochemicals, and biological warfare agents.
 17. Thesystem of claim 13, wherein the IR source is a pulsed laser, continuouslaser, broad band light source, filtered broad band light source, sweptsource, chirped source, variable source, or tunable source.
 18. Thesystem of claim 13, wherein multiple IR wavelengths are used to detectmultiple analytes of interest.
 19. The system of claim 13, whereinmultiple IR wavelengths are used to improve selectivity where at leastone IR wavelength is on-resonance for the analyte and at least one IRwavelength is off-resonance.
 20. The system of claim 13, wherein the IRsource emits somewhere in the range between 1 and 20 microns.
 21. Thesystem of claim 13, wherein to detect nitrogen based explosives, the IRsource is tuned to a wavelength at or near a peak in the absorptionspectrum characteristic of an N—O bond; wherein to detect phosphonateester chemical warfare agents, the IR source is tuned to a wavelength ator near a peak in the absorption spectrum characteristic of a P═O doublebond or a C—O—P bond; and wherein to detect drugs of abuse, the IRsource is tuned to a wavelength at or near a peak in the absorptionspectrum characteristic of a secondary or tertiary amine.
 22. The systemof claim 13, wherein an optical technique is used to examine the vaporplume.
 23. The system of claim 13, wherein the vapor plume is examinedusing an IR source, visible source, IR detector, visible detector,diffraction grating, filter wheel, Michelson interferometer, lasercavity and integrating sphere, or any combination thereof; and whereinthe presence of the analyte is determined by using IR absorption, IRbackscattering, IR thermal luminescence, Raman spectroscopy, LIBS, orany combination thereof.
 24. The system of claim 13, wherein the vaporplume is examined by ion mobility spectrometry, mass spectrometry, gaschromatography, chemiluminescence, surface acoustic wave, gravimetric,microbalance, crystal resonance, or any combination thereof.